1. Field of the Invention
The present invention relates to gas sensor elements with improved durability, superior waterproof properties, and improved strength, which is capable of detecting a concentration of a specific gas component contained in a target detection gas, for example, an exhaust gas emitted from an internal combustion engine of a vehicle engine, and also relates to a gas sensor equipped with this gas sensor element.
2. Description of the Related Art
There is a gas sensor placed in an exhaust gas passage equipped with an exhaust gas purifying apparatus through which a target detection gas flows. For example, the target detection gas is an exhaust gas emitted from an internal combustion engine such as a vehicle engine. Such a gas sensor placed in the exhaust gas passage of the internal combustion engine detects a concentration of a specific gas component contained in an exhaust gas in order to control the combustion state of the internal combustion engine or the operation of the exhaust gas purifying apparatus. There are oxygen, nitrogen oxide, ammonia, and hydrogen as the specific gas component to be detected.
There is an oxygen sensor as the above gas sensor, for example. This oxygen sensor is equipped with a multilayered type gas sensor element which has a lamination structure comprised of a solid electrolyte body, a detection electrode layer, a porous diffusion resistance layer, a reference electrode layer, a reference electrode chamber formation layer, and an insulation base body. When the gas sensor is an oxygen gas sensor, the solid electrolyte body has an oxygen ion conductivity. For example, the oxygen gas sensor has a plate-like shape. The detection electrode layer is formed on one surface of the solid electrolyte body, and contacts with a target detection gas. The porous diffusion resistance layer is formed at the detection electrode layer side of the gas sensor. The target detection gas to be detected is passed through the porous diffusion resistance layer in the gas sensor. The reference electrode layer is formed on the other surface of the solid electrolyte body, and contacts with a reference gas. The reference gas chamber formation layer has a reference gas chamber, and is formed at the reference electrode layer side. The reference gas is introduced into the reference gas chamber. The insulation base body has an embedded heater therein. In the multilayered type gas sensor element having a lamination structure, the solid electrolyte body, the detection electrode layer, the porous diffusion resistance layer, the reference electrode layer, the reference electrode chamber formation layer, and the insulation base body are laminated as described above.
In general, the exhaust gas emitted from the internal combustion engine contains oil-containing components and sensor-poisoning materials, where the oil-containing components are P, Ca, Zn, Si, etc, and the poisoning materials are made of additives such as K, Na, and Pb which are added into gasoline as a fuel. Those poisoning materials such as K, Na, and Pb contaminate the detection electrode layer and the porous diffusion resistance layer in the gas sensor element. This contamination often causes deterioration of the response capability and an output error of the gas sensor,
In addition, the exhaust gas generally contains water vapor component. Condensing the water vapor contained in the exhaust gas generates drops of water, and the drops of water are often adhered onto the surface of the multilayered type gas sensor element in the gas sensor.
In general, such a multilayered type gas sensor element having the above a lamination structure is used under a high temperature condition of not less than 700° C. because of being heated by the embedded heater, for example, in order to show an ion conductivity of the solid electrolyte body to a specific ion.
There is thereby a possibility of breaking the multilayered type gas sensor element by the large thermal shock generated when drops of water contained in the target detection gas contact to the surface of the multilayered type gas sensor element.
There are known methods in order to avoid the generation of error detection which is caused by trapping poisoning materials contained in the is exhaust gas as the target detection gas. For example, a conventional method forms a poison trapping layer having a predetermined thickness on the surface of the diffusion resistance layer. This diffusion resistance layer is exposed to the target detection gas (as disclosed in Japanese patent laid open publication No. JP 2006-126077).
Another conventional method forms a porous protective layer having a predetermined thickness on the outer peripheral surface of the gas sensor element in order for a drop of water to diffuse through the inside of the porous protective layer. This structure relaxes a thermal shock and avoids generation of cracks even if the drop of water (or the water drop) contacts to the surface of the gas sensor element (as disclosed in Japanese patent laid open publication No. JP 2007-121323).
However, because a poison trapping layer in the conventional multilayered gas sensor element described above is made of heat resisting particles. The heat resisting particles have a relatively large specific area in order to certainly trap poisoning materials contained in the target detection gas. Because the conventional multilayered gas sensor element easily absorbs the drop of water and thereby causes a lack of dry condition, this structure causes breaking of the gas sensor element when heated at a high temperature in order to activate it.